120 VEGETABLE FORCING 
Inasmuch as plant lice have sucking instead of biting 
mouth parts, they cannot be killed by stomach poisons 
such as arsenate of lead, but must be destroyed by con- 
tact insecticides or by suffocating fumigants. Tobacco 
extract and soap solutions are the most commonly 
applied of the liquids, and tobacco fumigation is uni- 
versally regarded as the most desirable means of con- 
trolling the green fly, especially on lettuce. See page 224. 
White fly (Aleyrodes vaporariorum).—The green- 
house white fly is widely distributed among establish- 
ments devoted to vegetable forcing. It is universally 
regarded as one of the worst foes of greenhouse crops, 
and must be combated in an intelligent and vigorous 
manner in order to prevent the most serious ravages. 
Tomatoes and cucumbers suffer most from the depreda- 
tions of the white fly. Lettuce, eggplant, bean, melon 
and many floral crops are subject to attack. 
A complete description and life history of the white 
fly (Fig. 40), and remarks on the appearance of infested 
plants are given as follows in Circular 57, U. S. Bureau 
of Entomology: 
“The mature white flies of both sexes are four-winged insects 
scarcely more than 114 millimeters or three-fiftieths of an inch in 
length. The adult white flies, as well as the scalelike larve, are 
provided with sucking mouth parts. In a short time after the 
emergence of the adult from the pupa case, the body, legs and wings 
become covered with a white, waxy substance which gives this, as 
‘well as other species of the genus, a characteristic floury appearance. 
The adults feed nearly continuously during their existence. If de- 
prived of food, they will rarely live for a longer period than three 
days under ordinary temperature conditions. The longest recorded 
length of life of one of these insects in the adult condition is 36 
days, but it seems probable that the average length of adult life is 
much greater than this would indicate. The largest number of eggs 
which an adult white fly is positively known to have deposited is 
129, but this number is probably below the average. Indeed, the 
specimen which produced this number of eggs with little doubt de- 
posited over 50 others which were not recorded. The number of 
